An “Effective functional subunit size” model for the dose response of rat spinal cord paralysis

• Background: Radiobiological models for normal tissue complication probability (NTCP) are more and more commonly used in order to estimate the clinical outcome of radiation therapy. A normal tissue complication probability model to be considered a good and reliable one should fulfill the following two requirements: (a) it should predict the sigmoid shape of the dose-response curve as well as possible and (b) it should duly handle the volume effect. In the work from 2005 (IJROBP 61(3):892-900, 2005) P. van Luijk et al. suggest that none of the existing NTCP models is able to describe the volume effects present in the rat spinal cord during irradiation with small proton beams and they indicate the need for developing such new models.Methods: We have used the experimental data from H. Bijl et al. (IJROBP 52(1):205-211, 2002) to try explaining the change in the fifty percent effective dose (ED50) for different field sizes. We initiated this study to evaluate whether the induction of white matter necrosis in rat spinal cord after irradiation with small proton beams could be explained independent of used NTCP model. We therefore introduced a new concept of effective FSU dose, where a convolution of the original dose distribution with a function describing the effective size of a single FSU results in the average doses in a functional subunit. Such procedure allows determining the ED50 in an FSU of a certain size, within the irradiation field. We have also looked at non uniform dose distributions to see whether using a similar method we can explain the so called “bath and shower experiments” (IJROBP 57(1): 274-281, 2003).Results: Using the least square method to compare the effective doses for different sizes of functional subunits with the experimental data we observe the best fit for about 8 mm length. It seems that this length could be understood as an effective size of functional subunits in rat spinal cord, explaining what is otherwise interpreted as a volume effect. For the non uniform dose distributions an effective FSU length of 5 mm gives the optimal fit with the Probit dose-response model.Conclusions: The concept of an effective FSU length seems to explain at least part of the effects seen when small portions of the rat spinal cord are irradiated. The most likely FSU length for the shower and bath experiments is 5 mm according to these calculations.

Subject headings

MEDICIN OCH HÄLSOVETENSKAP  -- Klinisk medicin -- Radiologi och bildbehandling (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Clinical Medicine -- Radiology, Nuclear Medicine and Medical Imaging (hsv//eng)

Keyword

Radiological physics

Radiofysik

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